Quad Flat No-leads (QFN) packages are commonly used for discrete devices due to the small footprint and the small package height.
An example of a semiconductor device 200 comprising a QFN package is shown in FIG. 1. The device 200 includes a lead frame 210 upon which is mounted a semiconductor substrate 202. A contact 206 on a surface of the substrate 202 allows an electrical connection to be formed with a first part of the lead frame 210 (e.g. by soldering). Another contact 204 is located on a surface of the substrate 202 opposite the surface on which the contact 206 is located. The contact 204 is electrically connected to another part of the lead frame using bond wire 250. The device 200 also includes an encapsulant 240 for protecting the features of the substrate 202 and lead frame 210. Parts 212 and 214 of the lead frame 210 are exposed at one side of the package 200 (i.e. they are not covered by the encapsulant 240). These parts 212 and 214 of the lead frame 210 provide external electrical connections for the device 200. For instance, the parts 212 and 214 of the lead frame 210 can be mounted on a carrier such as a printed circuit board (e.g. by soldering).
QFN packages have rather low heat capacity and heat conductivity, which can make then unsuitable for devices that dissipate a lot of power (such as like transient voltage suppression (TVS) protection devices). QFN packages can also add to the on-resistance of the device due to the bond wire that is used. Moreover, the thickness of QFN packages cannot be reduced significantly because lead-frame thickness, die thickness and bond wire loop height all add to the total thickness.
Devices such as transient voltage suppression (TVS) protection devices are used for protecting integrated circuits (ICs) against electrical overstress. In use, these devices are connected between an external input and the input of an IC, and can operate to drain unwanted, oftentimes large, currents to ground or another rail so that any internally provided protection of the IC is not overstressed and damaged.
The heat caused by the current within the protection device can limit the robustness of the device. The temperature within the protection device is dependent on factors such as the dissipated power, the thermal capacity of the device and the thermal resistance of the device.
TVS protection devices may include semiconductor diodes. In such a device, a pn junction may be provided near a first surface of a semiconductor substrate. In such devices, the main part of the heat created by the stress current occurs at the pn junction. On the other hand, the surface of the substrate is usually the part of the substrate that is most sensitive to heat (due to the presence of a metal contact on the surface, which can melt if the temperature in the device is too great). A second contact of the diode is usually provided on a second of the substrate. In such devices, heat diffusion from the pn junction to cooler parts of the device is highly asymmetrical because the pn junction is located near one of the surfaces of the substrate.
One kind of package for a semiconductor device involves the use of a clip bond. The clip replaces the bond wire in FIG. 1. The bottom of the substrate is soldered or glued to a lead frame finger, and a metal clip is soldered or glued to the top contact of the substrate and to a second lead frame finger. The two lead frame fingers serve as electrical contacts that can be soldered to a carrier such as a printed circuit board (PCB). However, a device using this approach may suffer from limitations associated with heat caused by stress pulses. For instance, the limited thickness of the clip limits its thermal capacity of the clip. The multiple solder or glue points can also limit thermal conduction away from the substrate. Lead frame materials are also not optimized for thermal resistance.
In another kind of package, known as the chip scale package, two contacts may be located on a common surface of the substrate. The substrate can be soldered top-down onto a carrier such as a PCB. The connections between the substrate and the carrier have limited thermal capacity. A further major disadvantage of the CSP is that current flow within the substrate flows in a lateral direction (between the two contacts on the top side). This can lead to current crowding and local heating, which can strongly reduce the robustness of the device.